Switch to using %r in the format string.
diff --git a/Parser/asdl.py b/Parser/asdl.py
index e8f9b1c..d6f5ec5 100644
--- a/Parser/asdl.py
+++ b/Parser/asdl.py
@@ -98,7 +98,7 @@
 
     def t_default(self, s):
         r" . +"
-        raise ValueError, "unmatched input: %s" % repr(s)
+        raise ValueError, "unmatched input: %r" % s
 
 class ASDLParser(spark.GenericParser, object):
     def __init__(self):